Pax6-dependent patterning in an annelid informs the evolution of bilaterian nerve cords

Conserved dorsoventral patterning systems have been proposed as evidence for a common evolutionary origin of centralized nervous systems in Bilateria, yet functional evidence outside vertebrates and arthropods remains limited. Here, we investigated the role of pax6 in the annelid Platynereis dumerilii using a mutant carrying a 61 bp deletion in the paired-domain coding region. Loss of pax6 disrupted ventral neuroectodermal patterning at 34 hpf, causing a shift in nk2.2 expression, narrowing of the nk6 domain, and downregulation of pax3/7, while msx expression remained largely unaffected. These early patterning defects were followed by selective neuronal abnormalities at 48 hpf, including displacement of TrpH-positive serotonergic cells and loss of posterior hb9-positive motoneuron domains. By 6 dpf, additional defects were observed in TrpH, ChAT, VAChT, and nk2.2 expression, accompanied by severe disruption of ventral nerve cord morphology and loss of the characteristic rope-ladder architecture. Together, these findings identify pax6 as a key regulator linking dorsoventral progenitor patterning, neuronal subtype specification, and nervous system morphogenesis in Platynereis. Our results provide functional evidence that the conserved dorsoventral patterning network plays an essential role in annelid ventral nerve cord development and support the view that important components of bilaterian nervous system patterning predate the divergence of major animal lineages.